Typical industrial inductively coupled plasma (ICP) sources uses an arrangement of two flat or vertical coils to control the plasma profile center to edge (for example, with respect to a substrate being processed) and allow uniformity tunability at the substrate level during processing, for example, in etch applications. In typical vertical, two-coil arrangements where the currents are flowing in the same direction (to be denoted as “in phase”), due to the nature of the constructive interference in the electric field between the coils at the substrate level, an M-shaped etch rate profile exists which limits the overall uniformity that may be desired for advanced node technologies. The location of the peak of the M-shaped etch rate profile in the power coupling under the dielectric window can vary based on the arrangement of the coils. If the currents in the coils are driven in the opposite direction from each other (to be denoted as “out of phase”), destructive interference in the electric field takes place between the coils, thus creating a null region under the dielectric window that isolates the ICP source from the vacuum chamber. Through diffusion, the overall plasma profile at the substrate level can be flattened, eliminating or reducing the M-shape signature. In such a scenario, the plasma is being pushed out towards the chamber walls. Hence, the overall ion flux reaching the substrate might be reduced, causing lower etch rates and reduced throughput. Therefore, to increase the etch rate, higher RF power is needed in out of phase operation. As some applications suffer from the M-shape effect, which, due to high throughput requirements and/or limitation of the RF power supply max power, further requires high ICP power, the out of phase operation may require significantly higher power which might require RF generators and matches that are not commercially available and extremely expensive. Moreover, under certain conditions, out of phase operation might run into capacitive coupling (E Mode) and/or inductive coupling (H mode) instabilities which narrow the window of operation for such processes.
Accordingly, the inventors believe there is a need to mimic the out of phase operation without resorting to higher power and by achieving the same effect in a different manner. Thus, the inventors propose a way to reduce M shape mimicking the out of phase operation while keeping in phase currents by manipulating the power coupling profile (power absorbed by plasma under the dielectric window).